The present invention relates to a very-low-temperature fiberscope used in liquefied gas of very low temperature, such as liquefied natural gas, liquefied propane gas, liquid nitrogen, liquid helium, etc.
Some devices kept at very low temperature may often require observation of their surface conditions. Such devices include a superconductive magnet device. The superconductive magnet device generally comprises a cryostat, a superconductive coil housed in the cryostat, and liquid helium for cooling the coil to very low temperature.
In the superconductive magnet device, a quenching phenomenon, such that the superconductive coil is shifted from a superconducting state to a normal conducting state, may present itself from some cause. In this case, energy stored in the superconductive coil needs to be released quickly to protect the coil from burning. Conventionally, therefore, the superconductive coil is connected with a detection system for electrically detecting the quenching phenomenon, an energy recovery circuit being connected to both ends of the superconductive coil when an output is delivered from the detection system. In this arrangement, however, the energy recovery circuit is connected so late that the superconductive coil is very liable to be burned. In order to ensure that the superconductive coil is prevented from being damaged by the quenching phenomenon, therefore, it is to be desired that a symptom of the phenomenon is detected in advance so that the energy recovery circuit can be connected to the superconductive coil immediately on detection of the symptom.
Such a symptom may be perceived by production of air bubbles from the surface region of the coil, or by deformation of the coil itself. The quenching phenomenon takes place when the temperature of a part of the superconductive coil is increased slightly, triggering a sudden increase in the temperature of the entire coil. When the temperature of the coil is increased slightly in the initial stage of the quenching phenomenon, the liquid helium in contact with the surface of the coil is heated to produce air bubbles therein. Accordingly, presentation of the quenching phenomenon can be foreseen by observing the production of air bubbles near the surface of the coil. The quenching phenomenon may also be caused by a distortion of the superconductive coil resulting from some external force. Therefore, development of the quenching phenomenon may be expected when the start of coil deformation is observed. In other words, observation of the surface conditions of the superconductive coil enables us to foresee a subsequent development of the quenching phenomenon.
Such observation of the surface region of the superconductive coil requires an image transmission system be placed in the cryostat so that an image of the superconductive coil can be transmitted externally for observation. A fiberscope may be used as an image transmission system which comprises an image guide consisting of a plurality of optical fibers in a bunch, an objective lens unit attached to one end of the image guide and etc.
When using the fiberscope for the observation, the objective lens unit and part of the image guide are immersed in the liquid helium contained in the cryostat, the helium being of a very low temperature of, for example, about 4.2 K. If the fiberscope used is intended for normal-temperature use, freezing or dew condensation will, possibly, ensue on the end face of the image guide or the surfaces of the objective lenses, constituting a hindrance to satisfactory observation.